Lithium-ion batteries are used as electric storage systems for powering electric and hybrid electric vehicles. These batteries comprise a plurality of suitably interconnected electrochemical cells arranged to provide a predetermined electrical current at a specified electrical potential. In each such cell, lithium is transported as lithium ions from a negative electrode through a non-aqueous, lithium-containing, electrolyte solution to a lithium-ion-accepting positive electrode as an electronic current is delivered from the battery to an external load, such as an electric traction motor. A suitable porous separator material, infiltrated with the electrolyte solution and permeable to the transport of lithium ions in the electrolyte, is employed to prevent short-circuiting physical contact between the electrodes. Graphite has been used as a negative electrode material and bonded in a thin electrode layer on a copper current collector. During charging of the cells, lithium is inserted into the graphite (lithiation, forming LiC6, about 372 mAh/g) and extracted from the graphitic carbon during discharging (delithiation). A suitable particulate material for receiving and storing inserted lithium during discharge of each cell is used as the positive electrode material. Examples of such positive electrode materials include lithium cobalt oxide (LiCoO2), a spinel lithium transition metal oxide such as spinel lithium manganese oxide (LiMnXOY), a lithium polyanion such as a nickel-manganese-cobalt oxide [Li(NiXMnYCoZ)O2], lithium iron phosphate (LiFePO4), or lithium fluorophosphate (Li2FePO4F), or a mixture of any of these materials. Suitable positive electrode materials are often bonded as a thin layer to an aluminum current collector. The electrochemical potential of such lithium ion cells is typically in the range of about 2 to 4.5 volts.
The use of lithium-ion batteries to power electric motors in automotive vehicles has led to the need for higher gravimetric and/or volumetric capacity batteries. While graphitic carbon is a durable and useful lithium-intercalating, negative electrode material for lithium-ion cells, it has a relatively low capacity for such lithium insertion. Other potential negative electrode materials such as silicon (theoretical capacity, 3600 mAh/g, for Li15Si4) and tin (theoretical capacity, 992 mAh/g, for Li22Sn5) have much higher theoretical capacities than graphite for lithium insertion. However, the volume change of up to 300 volume percent for silicon during lithiation and delithiation processes leads to fracture of the active silicon material and/or loss of electrical contact with the conductive additives or the current collectors. And tin has the same problem of a large volume expansion upon lithiation, leading to rapid capacity degradation.
Lithium-sulfur batteries, like lithium-ion batteries, are rechargeable. They are also notable for their high energy density. The low atomic weight of lithium and the moderate atomic weight of sulfur enable lithium-sulfur batteries to be relatively light in weight. Like lithium-ion cells, the anode or negative electrode of the lithium-sulfur cell requires lithium. During lithium-sulfur cell discharge, lithium is dissolved into an electrolyte from the anode surface, transported in the electrolyte (e.g., a molten or liquid alkali metal polysulfide salt) through a porous separator to a cathode (positive electrode during cell discharge) which comprises a polysulfide (e.g., S8). Upon reaching the cathode, lithium atoms progressively reduce the polysulfide to a lithium sulfur composition (e.g., Li2S3). The chemical changes are reversed when the lithium-sulfur cell is recharged. The light weight and high energy density of lithium-sulfur cells make lithium-sulfur batteries good candidates for vehicle propulsion systems and other electrical energy consuming devices.
The basic mechanism responsible for the loss of capacity of a battery due to fracture of the electrode materials in its cells is the loss of electrical contact with conductive material and the creation of new surfaces, which irreversibly consume the active lithium to form new solid electrolyte interfaces. Both problems shorten the effective cycling capacity of a battery. There remains a need for a more effective way or material form of utilizing silicon or tin in negative electrodes of lithium-ion cells.